Chromatin is classified into two main groups, euchromatin, where the DNA is loosely packaged, accessible and generally, but not always, transcriptionally competent, and heterochromatin, where the DNA is tightly packaged, inaccessible and generally, but not always, transcriptionally silent.
Epigenetics controls at least some of the transition between these two chromatin states. There are at least two main epigenetic events: DNA methylation and histone modification. These events affect how the DNA is packaged and whether the DNA is active or silent with respect to transcription.